The present invention relates to a method and apparatus for performing horizontal continuous casting, which is capable of step-wise extracting a molten metal from a tundish, and in particular for continuously casting and molding thin plate-like billets, or such as L-shaped or H-shaped billets.
Generally, in a horizontal continuous casting apparatus, a tundish and a mold or mold assembly are air-tightly joined. Since the static pressure of a molten metal is large at the molten metal solidification starting point, it is easy to cast a billet having a circular section. In such a case it is not necessary to utilize secondary cooling water because of the large cooling effect in a mold. Thus, the space necessary for arranging an entire casting apparatus can be advantageously reduced.
In a prior art technology, such as disclosed in the Japanese Patent Publication (KOKOKU) No. 1-54146 (54146/1989), there is proposed a continuous casting method for a thin plate-like billet in which a central portion of a mold is formed to have a substantially elliptic shape. A molten metal fed into this portion from a tundish thereby casts a substantially elliptical billet. The billet thus obtained is then deformed into a plate shape in the mold.
In the described prior art, however, the feeding of the molten metal from the tundish to the mold becomes unstable in its temperature distribution and vertical cracks are likely to occur in the billet. Furthermore, the shape of the mold becomes complex, resulting in an increases machining cost. In addition, since a shell at the initial time of the solidification of the molten metal is formed in this elliptic shaped mold, defects may occur on the surface of the billet due to extraction resistance in the mold.
More specifically, the horizontal continuous casting is performed by feeding the molten metal stored in the tundish to the mold, and then cooling it to form a billet having at least a solidified outer peripheral surface portion and extracting the billet by an extraction apparatus installed on the downstream side of the mold. In such a horizontal continuous casting apparatus, the tundish and the mold are air-tightly joined as described before, so that the static pressure of the molten metal in the mold is large and there is a good contact between the mold and the solidified shell of the billet, thus being preferably applicable to a formation of a round billet. As described above, in the horizontal continuous casting method, since the tundish and the mold are air-tightly joined, there is adapted a step-wise extraction driving method having one operation cycle of "pulling.fwdarw.pause.fwdarw.push.fwdarw.back.fwdarw.pause" steps, which is different from a vertical continuous casting method in which the billet is extracted at a constant speed.
The mold utilized for such a horizontal continuous casting apparatus has a cylindrical shape and forms a billet by cooling thereby solidifying at least an outer surface of the molten metal fed into a hollow portion, i.e. cavity, of the cylindrical mold. The thickness of the solidified shell of the billet after being taken out from the mold gradually increases and the strength also increases accordingly. Therefore, a press-down process performed for molding the billet to a desired shape is performed during a stage in which the billet has a relatively thin solidified thickness and is relatively soft. Furthermore, in the continuous casting method of a high alloy steel or high carbon steel, there may occur cases in which a low melting point substance, such as carbon or sulfur, is concentrated at a casting center portion of a solidification end, resulting in the occurrence of a problem, such as breaking, at a rolling step after the continuous casting. In order to obviate this problem, segregation is prevented by discharging the concentrated low melting point substance to an upstream side by pressing it down at a point near the solidification completion (crater end).
For example, Japanese Patent Laid-Open ublication (KOKAI) No. 62-81255 (81255/1987) discloses a casting strand forging apparatus such as shown in FIG. 17, in which an anvil-type press-down device 300 is disposed on an upstream side of a billet extraction device, and the forging apparatus is installed on the ground. For this condition, the driving reaction force at the press-down formation time constitutes a load to the billet extraction device and, accordingly, it is necessary to enlarge the driving capacity of the billet extraction device. In such case, in a vertical continuous casting apparatus in which the billet is continuously cast at a constant speed, increasing the driving capacity of the extraction device does not constitute a large problem.
On the other hand, in a horizontal continuous casting apparatus in which the billet is step-wise extracted, since the solidificetion starts at a break ring end, the increasing the driving capacity of the extraction device adversely affects the quality of the billet. This is because, in a certain sense, the horizontal continuous casting apparatus generally utilizes a servo motor having a good performance, but there are few servo motors which have a good performance for large capacities. Furthermore, in the press-down device, the casting speed differs from that of the extraction device, which is a difficult problem for control, thus degrading the positioning performance and resulting in billet surface defects, i.e. cold-shut cracks are more likely. Further, the apparatus is increased in size because the apparatus is of the ground installation-type. It is difficult to maintain the accuracy of the pulling stroke and push-back stroke in the extraction device further degrading the quality of the billet.